Bulk synthesis of long nanotubes of transition metal chalcogenides

ABSTRACT

Nanotubes of transition metal chalcogenides as long as 0.2-20 microns or more, perfect in shape and of high crystallinity, are synthesized from a transition metal material, e.g. the transition metal itself or a substance comprising a transition metal such as an oxide, water vapor and a H 2 X gas or H 2  gas and X vapor, wherein X is S, Se or Te, by a two-step or three-step method. The transition metal chalcogenide is preferably WS 2  or WSe 2 . Tips for scanning probe microscopy can be prepared from said long transition metal chalcogenide nanotubes.

[0001] The present application is a division of copending parentapplication Ser. No. 09/959,664 filed Jan. 28, 2002, which itself is thenational stage under 35 U.S.C. 371 of the international applicationPCT/IL00/00251, filed May 2, 2000, which designated the United States,and which international application was published under PCT Article21(2) in the English language.

FIELD OF THE INVENTION

[0002] The present invention relates to methods for the bulk synthesisof long nanotubes of transition metal chalcogenides and to methods forpreparation of tips for scanning probe microscopy from said longnanotubes.

BACKGROUND OF THE INVENTION

[0003] The discovery of carbon nanotubes in 1991 (Iijima, 1991) hasgenerated intense experimental and theoretical interest over the lastfew years because of their unusual geometry and physical properties.Besides the original carbon structure, similar inorganic structures havealso emerged: BN (Chopra et al., 1995), V₂O₅ (Ajayan et al., 1995), MoS₂(Feldman et al., 1995; Remskar et al., 1998 and 1999a; Zelensky et al.,1998); and WS₂ (Tenne et al., 1992; Remskar et al., 1998, 1999a and1999b). The reason for such an analogy between the pure carbon andinorganic structures is based on the fact that they all stem fromlamellar (2D) compounds.

[0004] The case of the layered transition-metal dichal-cogenides (WS₂and MoS₂) was the first example of such an analogy. Indeed, in 1992, IF(inorganic fullerene-like) structures and nanotubes of WS₂ were reportedby the laboratory of the present inventors (Tenne et al., 1992; EP0580019; U.S. Pat. No. 5,958,358), followed shortly by similar resultson MoS₂ (Margulis et al., 1993) and the respective selenides(Hershfinkel et al., 1994). However, it is noteworthy to underline thatthe samples contained minute amounts of IF particles. Instead, most ofthe samples consisted of WS₂ platelets (2H-WS₂). The nanotubes wererelatively rare and constituted even a smaller fraction of the totalcomposition. Besides this statistical fact, the reproducibility of thenanotubes growth was rather poor. Consequently, a lot of effort has beenrecently devoted to the study of nanotubes from new other relatedmaterials.

[0005] None of the methods described recently for the synthesis of WS₂and MoS₂ nanotubes mentioned above permit synthesis of bulk quantitiesof a single phase of inorganic nanotubes and mostly perfect inorganicnanotubes to be obtained.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide methods forthe bulk synthesis of inorganic nanotubes, particularly of longnanotubes of transition metal chalcogenides.

[0007] In one aspect, the invention relates to a two-step method forbulk synthesis of long nanotubes of transition metal chalcogenides froma transition metal material, water vapor and a H₂X gas or H₂ gas and Xvapor, wherein X is S, Se or Te, said method comprising:

[0008] a) either heating a transition metal material in the presence ofwater vapor in a vacuum apparatus or electron beam evaporating atransition metal material in the presence of water vapor, at a suitablepressure, to obtain nanoparticles of the transition metal oxide as longas 0.3 microns; and

[0009] b) annealing the transition metal oxide nanoparticles obtained instep (a) in a mild reducing atmosphere with a H₂X gas or H₂ gas and Xvapor, wherein X is S, Se or Te, at a suitable temperature, in order toobtain long nanotubes of the transition metal chalcogenide.

[0010] In alternative routes, in order to obtain larger nanotubes,either a foil of the transition metal is heated in poor vacuumconditions (e.g. 1 Torr) or nanoparticles of the transition metal oxideas large as 0.3 microns of step (a) are further elongated, to obtaintransition metal oxide whiskers/nanoparticles as long as 10-20 micronsor more, which are then annealed with the H₂X gas or with H₂ gas and Xvapor.

[0011] Thus, according to another embodiment, the invention relates to athree-step method for bulk synthesis of long nanotubes of a transitionmetal chalcogenide from a transition metal material, water vapor and aH₂X gas or with H₂ gas and X vapor, wherein X is S, Se or Te, saidmethod comprising:

[0012] a) either heating a transition metal material in the presence ofwater vapor in a vacuum apparatus or electron beam evaporating atransition metal material in the presence of water vapor, at a suitablepressure, to obtain nanoparticles of the transition metal oxide as largeas 0.3 microns;

[0013] b) elongating the transition metal oxide nanoparticles as largeas 0.3 microns of step (a) to obtain nanoparticles as long as 20 micronsor more; and

[0014] c) annealing the elongated transition metal oxide nanoparticlesobtained in step (b) in a mild reducing atmosphere with a H₂X gas orwith H₂ gas and X vapor, wherein X is S, Se or Te, at a suitabletemperature, in order to obtain long nanotubes of the transition metalchalcogenide.

[0015] The elongation of the transition metal oxide nanoparticles instep (b) can be carried out by any known and suitable method, forexample by heating the oxide under mild reducing conditions for a fewminutes such as for 5-30, preferably, 10 minutes, or by electron beamirradiation of the oxide in high vacuum conditions.

[0016] When a mixture of nanotubes of two different metal chalcogenidesis desired, for example metal sulfide and metal selenide, the annealingstep is carried out by alternating the annealing atmosphere, forexample, by alternating H₂S and H₂Se gas or by alternating the S and Sevapors in the presence of H₂.

[0017] The nanotubes obtained by the methods of the invention areperfect in shape and of high cristallinity and may be 0.2-20 micron longor more. For the sake of convenience, the nanotubes of the inventionshorter than 0.5 microns are sometimes herein in the specificationreferred to as “short” nanotubes to distinguish them from the longernanotubes.

[0018] The metal material may be the transition metal itself, a mixtureof or an alloy of two or more transition metals, a substance comprisinga transition metal, e.g. an oxide, and a mixture of substancescomprising two or more transition metals. Examples of transition metalsinclude, but are not limited to, Mo, W, V, Zr, Hf, Pt, Re, Nb, Ta, Ti,and Ru. The electron beam evaporation embodiment is more suitable forrefractory transition metals, e.g. Nb, V, Ta, Ti.

[0019] In one preferred embodiment, the invention relates to a two-stepmethod for bulk synthesis of long nanotubes of WS₂ and/or WSe₂ whichcomprises:

[0020] a) either heating W in the presence of water vapor in a vacuumapparatus, or electron beam evaporating W or WO₃ in the presence ofwater vapor, at a pressure of 1-20, preferably 8-12, Torr, thusobtaining WO₃ nanoparticles as large as 0.3 microns; and

[0021] b) annealing the WO₃ nanoparticles obtained in step (a) in a mildreducing atmosphere with H₂S or H₂Se gas or with H₂ and S or Se vapor,or by alternating the annealing atmosphere with H₂S and H₂Se or with H₂and S or Se vapor, at 800-850° C., preferably at 835-840° C., thusobtaining relatively long and hollow WS₂ and/or WSe₂ nanotubes as longas 10 microns or more.

[0022] Longer WS₂ and WSe₂ nanotubes can be obtained when in step (a) aW foil is heated in poor vacuum conditions, e.g. of 1 Torr, and the longtungsten oxide whiskers obtained are then annealed sulfidized orselenized.

[0023] In another preferred embodiment, the invention relates to athree-step method for bulk synthesis of long nanotubes of WS₂ and/orWSe₂ which comprises:

[0024] a) either heating W in the presence of water vapor in a vacuumapparatus or electron beam evaporating W or WO₃ in the presence of watervapor, at a pressure of 1-20, preferably 8-12, Torr, thus obtaining WO₃nanoparticles as large as 0.3 microns;

[0025] b) heating the WO₃ nanoparticles as long as 0.3 microns undermild reducing conditions at 800-850° C., preferably at 835-840° C., forabout 10 minutes to obtain WO₃ nanowhiskers as long as 10 microns; and

[0026] c) annealing the WO₃ nanoparticles obtained in step (b) in a mildreducing atmosphere with H₂S or H₂Se gas or with H₂ and S or Se vapor,or by alternating the annealing atmosphere with H₂S and H₂Se or with H₂and S or Se vapor, at 800-850° C., preferably at 835-840° C., thusobtaining relatively long and hollow WS₂ and/or WSe₂ nanotubes as longas 10 microns or more.

[0027] The mild reducing conditions for elongation of the oxidenanoparticles in step (b) of the three-step method include, for example,heating the oxide nanoparticles under the flow of H₂ (0.05-1.0%)/N₂(99.95-99%)−110 ml/min gas stream for up to 10 minutes. Under theseconditions, elongation of the oxide nanoparticles is achieved. Withhigher amounts of H₂, e.g. above 5% H₂, no elongation is obtained.

[0028] The mild reducing atmosphere for annealing the oxidenanoparticles includes, for example, sulfidization or selenization underthe flow of H₂ (1%)/N₂ (99%)−110 ml/min and H₂S−1 ml/min. If the flow ofH₂S is lower than 1 ml/min, then longer transition metal chalcogenidenanotubes are obtained.

[0029] The invention further relates to long transition metalchalcogenide nanotubes as long as 20 microns or more obtained by amethod of the invention. In one embodiment, said transition metalchalcogenide is WS₂ and/or WSe₂.

[0030] The invention additionally relates to tips for scanning probemicroscopy (both STM and FTM) and methods for preparation of such tipsfrom the long transition metal chalcogenide nanotubes obtained by themethods of the invention, comprising:

[0031] a) transferring adhesive from carbon tape to a microfabricated Sitip; and

[0032] b) pulling off bundles of said transition metal chalcogenide longnanotubes with this tip from a mat of nanotubes prepared on a differentarea of the tape.

BRIEF DESCRIPTION OF THE FIGURES

[0033]FIG. 1a is a typical TEM image of WO_(3-x) needles 40 nm in length(the scale bar represents 100 nm); FIG. 1b shows hollow WS₂ nanotubesobtained from the asymmetric oxide nanoparticles shown in FIG. 1a (thescale bar represents 20 nm).

[0034] FIGS. 2A-2B are SEM micrographs of a mat of WS₂ nanotubes (6-7sulfide layers) with an oxide core at 2 different magnifications (2A—50μm; 2B—10 μm). The inset in FIG. 2A shows a typical hollow nanotubeobtained after completion of the sulfidization process (the scale barrepresents 20 nm). Distance between each two WS₂ layers (fringes) is0.62 nm.

[0035]FIG. 3 shows Raman spectra of: curve A. WO₃; curve B. WO_(2.9)long nanowhiskers; curve C. WO₂ powder; curve D. WS₂ nanotubes.Excitation source used: 1 mW 632.8 nm laser beam.

[0036]FIGS. 4a-c show TEM micrographs and the corresponding ED patternsof tungsten oxide (WO₃) particles synthesized at different water vaporpressures: (a) P_(H2O)=5 Torr (scale bar—50 nm); (b) P_(H2O)=12 Torr(scale bar—50 nm); (c) P_(H2O)20 Torr (scale bar-20 nm).

[0037]FIG. 5 shows TEM micrograph of “short” WS₂ nanotubes with oxide inthe core.

[0038]FIG. 6a shows SEM micrographs of long hollow or oxide-free WS₂nanotubes at two different magnifications (upper figure—2 μm; lowerfigure—500 nm). FIG. 6b shows TEM micrograph of long WS₂ nanotubes(scale bar—1 μm).

[0039]FIG. 7 shows TEM micrographs of: (7 a) apex of one WS₂ nanotubesynthesized by the “three-step method”; (7 b) nanotube walls, whichcontain several defects.

[0040] FIGS. 8A-8C depict a schematic illustration of the growth processof the encapsulated sulfide/oxide nanowhisker. 8A. Initialization of thesulfidization process of the asymmetric oxide nanoparticle; 8B. Growthof a long sulfide/oxide encapsulated nanowhisker. 8C. [010] is thegrowth axis of the oxide whiskers.

[0041] FIGS. 9A-9D show a comparison of microfabricated sharp Si tip(NT-MDT) and WS₂ nanotube tip for measuring deep structures (nominal 670nm) of varying linewidth. 9A and 9B microfabricated S₁ and WS₂ nanotubetips, respectively, on 350 nm linewidth structure; 9C and 9D—600 nmlinewidth structure. Note that in case of FIG. 9A, the Si tip cannotreach the bottom of the trench, while the nanotube in FIG. 9B is able tofollow the trench contour faithfully.

DETAILED DESCRIPTION OF THE INVENTION

[0042] According to the present invention, the synthesis of a pure phaseof very long and hollow WS₂ nanotubes from short but asymmetric oxidenanoparticles was achieved. In this process, the oxide nanoparticlegrows along its longest axis; and subsequently its outermost layer isbeing sulfidized, while the growing oxide tip remains uncoated as longas the nanowhisker continues to grow. Thereafter, a superlattice of{001}R crystal shear is formed in the oxide core, and the diffusioncontrolled sulfidization of the oxide core is completed within 60-120min.

[0043] In one embodiment, the synthesis of WS₂ nanotubes involves twosteps, each one carried-out in a separate reactor: first, W is heated inthe presence of water vapor in a vacuum apparatus or W or WO₃ iselectron beam evaporated in the presence of water vapor, at a pressureof 1-20 Torr, and then the thus obtained WO₃ nanoparticles as large as0.3 microns are then reacted with H₂S gas under mild reducingconditions.

[0044] In order to obtain longer WS₂ nanotubes, a three-step can becarried out in which an intermediate step is added for the elongation ofthe WO₃ nanoparticles as large as 0.3 microns before they are reactedwith H₂S gas under mild reducing conditions.

[0045] The simultaneous reduction and sulfidization reactions were foundto be essential for the encapsulation process, which is the key step inthe formation of nested fullerene-like WS₂ structures from oxidenanoparticles (Feldman et al., 1998). While sulfidization of thesulfide/oxide composite nanoparticle proceeds, more sulfide layers arebeing added from the outside inwards. Concomitantly, the remaining oxidecore is further reduced and gradually transforms into an orderedsuperlattice of {001}R CS planes. These planes, which stretch along thewhisker's growth's axis can be easily observed by TEM since they presentstrong contrast modulation. This microscopic structure is a directmanifestation of the reduction process, which affects the homologousseries of tungsten suboxides phases—W_(n)O_(3n−1) (Miyano et al., 1983).

[0046] A great advantage of the process of the present invention is theabsence of almost any contaminant or byproduct. It is also remarkablethat no catalyst, which must be separated and dislodged from thenanotube mat at the end of the growth, is necessary in the currentprocess. Therefore, tedious purification steps to isolate the nanotubes,which are time consuming and expensive, are not required once theprocess is completed.

[0047] Similar conditions as described herein for the bulk synthesis oflong WS₂ nanotubes can be used for carrying out the bulk synthesis ofother transition metal chalcogenide nanotubes.

[0048] The transition metal chalcogenide nanotubes of the invention canbe used for the preparation of tips for scanning probe microscopy bymethods well-known in the art such as the procedure of Dai et al., 1996.Using these tips, images of high aspect ratio replica and evaporated Tifilms with sharp asperities became feasible, which could not be achievedwith commercially available sharp Si tips.

[0049] The invention will now be illustrated by the followingnon-limiting Examples.

EXAMPLES Example 1 Synthesis of WS₂ Nanotubes (Experiment A)

[0050] 1a. Synthesis of Precursor Nanoparticles of WO_(2.9)

[0051] In the first step (stage I), a powder consisting of asymmetricoxide nanoparticles of ca. 10-30 nm in diameter and a length of 40-300nm is produced in a high vacuum evaporator. After pumping to ˜10⁻⁴ Torr,water vapor from an external reservoir is introduced through a needlevalve, while pumping with a rotary vane pump so that the pressure can beregulated to any desired value up to the vapor pressure of water at roomtemperature, ca. 20 Torr. A tungsten filament is heated to 1600±20° C.The water molecules react with the hot tungsten filament and produce(WO₃)_(n) clusters which condense on the walls of the bell jar or,alternatively, onto a water-cooled copper surface. If the water vaporpressure is maintained in the pressure range 8-12 Torr, crystallineoxide nanoparticles with an asymmetric shape are produced. Atransmission electron microscope (TEM) image of a typical batch ofnanoparticles produced at 12 Torr, is shown in FIG. 1a. Lower vaporpressures (<5 Torr) yield amorphous nanoparticles of a non-defined(spaghetti-like) or spherical shape. It is also noted that the powdercolor varies with the water vapor pressure and is an excellent indicatorfor the deviation from stoichiometric yellow-green WO₃ phase. Whereas athigh vapor pressure (˜20 Torr) a powder in light blue color, which isidentified by electron diffraction (ED) as a mixture of WO₃ and WO_(2.9)is obtained, a deep blue WO_(2.9) phase is accrued under lower watervapor pressure.

[0052] 1b. Synthesis of Long Nanotubes of WS₂ (Two-step Method)

[0053] The tungsten oxide powder of Example 1a was collected and 50 mgthereof were transferred to another reactor, in which sulfidizationunder controlled temperature (835-840° C.), and N₂/H₂+H₂S gas flow takesplace. Sulfidization of oxide nanocigars ca. 40 nm long under mildreducing conditions, i.e. N₂(99%)/H₂(1%)−110 ml/min and H₂S−1 ml/min,lead to the formation (ca. 40 mg) of relatively long or “short”nanotubes of WS₂ (˜0.2-0.5 μm), as shown in FIG. 1b.

[0054] When the precursor oxide consisted of nanoparticles 100-300 nm inlength (FIG. 1a), nanotubes as long as 10 microns were obtained. FIG. 2shows typical scanning electron microscope (SEM) images of this phase attwo different magnifications, showing a mat of nanotubes consiting of 6sulfide layers with an oxide core. The formation of hollow WS₂ nanotubesfrom oxide nanowhiskers was followed by both SEM and TEM imaging. Whileno discrenible changes in the overall shape of the nanoparticles couldbe observed by the SEM, the detailed microscopic changes, during theoxide to sulfide conversion, could be easily followed by the TEM work.

[0055] TEM micrograph of the apex of a long and hollow WS₂ nanotube,obtained by this method, is shown in the inset of FIG. 2A. While thecross section of the nanotubes is quite similar to that of the precursornanoparticles, their length increases by a factor of 20-40 during thesulfidization step. Note, however, that about 10% of the nanotubes havean enlarged cross-section diameter of app. 100 nm.

[0056] 1c. Elongation of Precursor Nanoparticles of WO_(2.9) (Stage IIof Three-step Method)

[0057] In another series of experiments (stage II), app. 5 mg of theamorphous spaghetti-like oxide nanoparticles, were heated to 835-840° C.under the flow of H₂(1%)/N₂(99%)−110 ml/min gas stream for 10 min. Thisprocess yielded a mat of oxide nanowhiskers, typically 10 μm long and20-50 nm thick. About 80% of the oxide nanowhiskers, obtained in thisway, were thin (ca. 30 nm) and cylindrical in shape. The rest, did nothave a circular but rather a rectangular cross section (ca. 10×100 nm).Furthermore, they were completely crystalline and the prevailing oxidephase was identified by ED as WO_(2.9). TEM of the nanowhiskers revealed{10∞}={001}R crystal shear (CS) planes along the [010] growth axis ofthe nanowhiskers, but the CS planes were not equally spaced occasionallyhowever, needles with an ordered CS superlattice were obtained. If theprocess was overextended, complete reduction of the oxide into tungstenmetal nanorods was observed.

[0058] 1d. Synthesis of Long Nanotubes of WS₂ (Stage III of theThree-step Method)

[0059] In the next step (stage III), the elongated oxide nanowhiskers of1c above were heated at 835-840° C. under the flow of a gas mixtureconsisting of H₂S (2 ml/min) and N₂ (110 ml/min) for 120 min yieldingabout 4 mg of WS₂ nanotubes with characteristics very similar to the onepresented in FIG. 2. Nonetheless, a substantial fraction (ca. 20%) ofthe material, obtained in this way, formed non-perfectly closednanotubes. Selenization of the same oxide nanowhiskers lead to theformation of very long WSe₂ nanotubes, and mixed WS₂/WSe₂ nanotubes werealso prepared by alternating the annealing atmosphere with H₂S and H₂Se.

[0060] 1e. Characterization of Precursor WO_(2.9) and of WS₂ by RamanSpectra

[0061] The Raman spectra at different stages of the oxide nanowhiskergrowth and reduction were measured and are shown in FIG. 3. The spectrumof a pure WO₃ powder (curve A) is in good agreement with previousstudies (Horsley et al., 1987). The signal of the WO_(2.9) precursors,which are X-ray amorphous, consisted of an intense background,indicating that these oxide nanoparticles are indeed amorphous.Unfortunately, the strong absorption and Raman scattering of the sulfideenvelope hides the Raman pattern of the oxide core, in the partiallysulfidized nanowhiskers. In order to gain some insight into thestructure of the reduced oxide nanowhiskers, its Raman spectrum wasmeasured. Curve B shows a spectrum of the 3-10 μm long oxidenanowhiskers (prepared in stage II), which have partially ordered shearstructure {001}R. The Raman spectrum of WO₂, which was not reportedhitherto, is shown in curve C. Finally, the Raman spectrum of the WS₂nanotubes (prepared in stage III) coincides with that of crystalline2H—WS₂ (curve D) (Frey et al., 1998).

[0062] The absence of data in the literature for the Raman of thereduced WO_(3-x) (0<x≦1) phases probably reflects the difficulty toprepare the pure phases of the different suboxides (however vide infra).It is important to note, that although the suboxide has anon-stoichiometric composition, it produces a distinct Raman spectrum.The appearance of new peaks in the Raman spectrum of the suboxide (curveB) reflects the appreciable distortion in the WO₆ octahedra in thisphase. Using an empirical formula described by Hardcastle et al., 1995,it is possible to associate the 870 cm⁻¹ mode in curve b with thestretch mode of a 1.78 Å W—O bond. This value compares favorably withthe calculated 1.77 Å for one of W—O bondlengths in the W₃O₈ structure.The new bands in the W—O bending region (200-400 cm⁻¹) are attributed tothe fact that each nanowhisker contains at least one of several possiblemembers of the W_(n)O_(3n−1) homologous series, with different CSdistance. The most intense peak in the Raman spectra of WO₂ (curve C)appears at 285 cm⁻¹, and is assigned to the W—O—W bending mode, whichappears in 275 cm⁻¹ for WO₃ (curve A). Thus, the shift of this band to ahigher frequency is attributed to the constrained W—O—W bending in themore compact distorted-rutile structure of WO₂. It is also important tonote that no Raman bands around 950 cm⁻¹, indicative of hydratedclusters, could be discerned.

[0063] Hence, the present Raman measurements strongly indicate theformation of partially ordered CS planes in the reduced oxidenanowhiskers (stage II). This superstructure is likely to be animportant intermediate stage in the formation of WS₂ nanotubes.

Example 2 Synthesis of WS₂ Nanotubes (Experiment B) 2a. ExperimentalSection

[0064] (i) Synthesis of the WO_(3-x) Particles

[0065] Tungsten suboxide particles (WO_(3-x)) were produced by heating atungsten filament (model ME11 from the R. D. Mathis company) in thepresence of water vapor inside a vacuum chamber, by the followingprocedure: once the vacuum in the bell-jar had reached a value of 10⁻⁴Torr, the filament was heated for a few minutes in order to remove thesuperficial oxide layer. Water vapor was then allowed to diffuse intothe vacuum chamber through an inlet, until the desired pressure wasreached. The filament was heated to around 1600±20° C., while thepressure in the chamber was maintained constant during the evaporationprocess (a few Torr). After a few minutes of evaporation, a blue powdercondensed on the bell-jar walls. The accrued powder consisted ofneedle-like WO_(3-x) particles (ca. 50 nm in length and 15 nm indiameter) under a specific water vapor pressure.

[0066] NiCl₂ or CoCl₂ (2×10⁻³ M) salts were dissolved in the waterreservoir before each evaporation. The nanoparticles produced in thepresence of the transition-metal salt appeared to be more crystallinethan those obtained without the addition of a salt, as shown by ED(electron diffraction).

[0067] (ii) Synthesis of the WS₂ Nanotubes Starting From the WO_(3-x)Nanoparticles

[0068] The synthesis of the WS₂ nanotubes starting from the needle-likeWO_(3-x) particles was done in a reactor similar to the one used for thesynthesis of IF-WS₂ particles (Feldman et a., 1996, 1998). The principleof the synthesis is based on a solid-gas reaction, where a smallquantity (5 mg) of WO_(3-x) particles (solid) is heated to 840° C. underthe flow of H₂/N₂ (forming gas)+H₂S gas mixture. In order to avoidcross-contamination between the different runs and minimize memoryeffects, which can be attributed to the decomposition of H₂S anddeposition of sulfur on the cold walls of the reactor, flushing of thereactor (10 min) with N₂ gas flow was performed after each synthesis.

[0069] Samples were studied using a scanning electron microscope (SEM)(Philips XL30-ESEM FEG), a transmission electron microscope (TEM)(Philips CM 120 (120 keV)) and X-ray diffraction (XRD) (Rigaku RotaflexRU-200B) having Cu—Kα anode. The electron diffraction (ED) patterns wereobtained on a high resolution transmission electron microscope (HRTEM)(JEM-4000EX) operated at 400 kV. Ring patterns from TiCl were used as acalibration reference standard for the ED patterns. The accuracy of thed-spacings was estimated at ±0.005 nm.

[0070] 2b. Synthesis of Tungsten Oxide Needle-like Nanoparticles (StageI)

[0071] Three different values of water vapor pressure were selected:P_(H2O)=5, 12 and 20 Torr, the latter corresponding to the thermodynamiclimit of the water vapor pressure at room temperature (22° C.). Thetexture of all the batches appeared to be more or less the same after afew minutes of evaporation. However, a variation in the color of thepowder, which was collected on the walls of the bell-jar, was noticed. Acolor range, which goes from dark blue for P_(H2O)=5 Torr to light bluefor P_(H2O)=20 Torr, was observed.

[0072] The water vapor pressure in the chamber apparently influences themorphology and the stoichiometry of the nanoparticles obtained byevaporation. For a low value (P_(H2O)=5 Torr), the oxide nanoparticlesdid not have a well-defined morphology (FIG. 4a). The ED patternconfirms that the powder is completely amorphous (not shown). When thepressure was increased (P_(H2O)=12 Torr) the nanoparticles presented acylindrical shape and were crystalline. A typical batch is shown in FIG.4b, where the dimensions of the whiskers are typically around 50 nm inlength and 15 nm in diameter. For the thermodynamic limit of the waterpressure at room temperature (P_(H2O)=20 Torr), a growth in bothdirections (along the nanoparticle long axis and perpendicular to it)led to the formation of needle-like particles with much smaller aspectratio and steps perpendicular to the long axis. The whiskers arecrystalline as could be evidenced from the ED pattern, which is similarto the one observed for the particles produced at 12 Torr (FIG. 4c).

[0073] The stoichiometry of the particles could not be easily assignedby XRD for several reasons. First, most of the samples were notsufficiently crystalline for generating well defined peaks in thespectrum. Moreover, several nonstoichiometric tungsten oxide phases havebeen reported in the literature and all of them exhibit very similarpatterns. Consequently, assigning the stoichiometry of the concernedphase accurately from the XRD data, was rather difficult. Themeasurement by electron diffraction of a bundle of individualneedle-like crystals was more informative in this case. The values ofthe d_(hkl) spacings were calculated for the crystalline whiskerssynthesized at P_(H2O)=12 Torr and at P_(H2O)=20 Torr. Both sets ofwhiskers can be interpreted as having an average substructure similar tothat of the reported tetragonal phase W₂₀O₅₈ (WO_(2.9)) originallydescribed by Glemser et al. (1964) and the results are shown in Table 1.The needles can be described according to a substructure of WO₃interspersed with defects attributable to random crystallographic shearplanes occurring either parallel to the needle axis or, alternatively,at some angle to the beam direction as the needles are viewed in theHRTEM. Further evidence of the randomness of the defects occurring inthe needles is given by the prominent diffuse streaking that is oftenobserved in ED patterns obtained from these needles (Sloan et al.,1999). It is noteworthy to underline that, whatever the pressure insidethe chamber, the batches appeared to be homogeneous in their morphology,providing needle-like particles of relatively constant oxidestoichiometry for a given preparation.

[0074] A detailed study of the conditions required for the whisker'sgrowth was then undertaken, the role of the water in this process beingexamined first.

[0075] 2c. The role of Water in the Tungsten Oxide Whisker's Growth

[0076] To study the role of water in the oxidation of the tungstenfilament, evaporations were performed with oxygen instead of water vaporin the chamber. Indeed, oxidation of the tungsten filament could beperformed either with water vapor according to equation 1 below or withpure oxygen according to equation 2, both reactions being exothermic inthe conditions of the present measurements (temperature of the filament:1600±20° C., and pressure in the chamber maintained at 12 Torr). Thefree energies of the reactions were calculated using the data describedin Horsley et al., 1987, for STP (standard) conditions. $\begin{matrix}{{\left. {{W(s)} + {3H_{2}O\quad (g)}}\rightarrow{{{WO}_{3\quad}(s)} + {3H_{2}\quad (g)}} \right.,{{\Delta \quad G_{({{1873\quad K\quad {and}\quad P} = {12\quad T}})}} = {{- 21}\quad k\quad J\quad {mol}^{- 1}}}}\quad} & \lbrack 1\rbrack \\{\left. {{W(s)} + {{3/2}\quad O_{2}\quad (g)}}\rightarrow{{WO}_{3\quad}(s)} \right.,{{\Delta \quad G_{({{1873\quad K\quad {and}\quad P} = {12\quad T}})}} = {{- 150.5}\quad {kJ}\quad {mol}^{- 1}}}} & \lbrack 2\rbrack\end{matrix}$

[0077] To perform the evaporation with the same quantity of oxygen asfor the one performed in the presence of water vapor, the oxygenpressure was maintained at P_(O2)=6 Torr compared to P_(H2O)=12 Torr(n_(O2)=½n_(H2O)). The resultant particles were 100% spherical orfaceted, typically 5 to 30 nm in diameter. The color of the powder waslight blue, which can be ascribed to a slight reduction of the powder bytraces of water still present in the vacuum chamber. When the oxygenpressure was decreased, light blue phases of spherical or facetednanoparticles were observed as well.

[0078] The absence of needle-like particles in presence of oxygen in thechamber is indicative of the role played by hydrogen in generating anasymmetric growth of the nanoparticles (see equations 1 and 2).

[0079] These findings allude to the fact that the needle's growthconsists of a two-step process occurring simultaneously on the hotfilament surface. The first step is the oxidation of the tungstenfilament, which leads to the formation of WO₃ particles. In the nextstep, reduction of these particles results in the formation of WO_(3-x)needle-like particles (eq 3).

WO₃ (s)+H₂ (g)→WO_(3-x) (g)+x H₂O (g)+(1−x)H₂ (g)  [3]

[0080] It is important to note that the direct reaction between watervapor and the W filament is not the only plausible oxidation route.Indeed two pathways could be contemplated for the oxidation of W withwater. The first one corresponds to the direct reaction of watermolecules with W atoms (eq 1). Alternatively, partial waterdecomposition (see eq 4) leads to the oxidation of the hot tungstenfilament by liberated oxygen.

H₂O (g)→½O₂ (g)+H₂ (g), ΔG ₍₁₈₇₃ K and P=12 T)+33.9 kJ mole⁻¹  [4]

[0081] Regardless of whether the direct or indirect mechanism iscorrect, H₂ is a resultant product of both reactions. It is thereforebelieved that hydrogen is involved in the production of needle-likeparticles as opposed to the spherical ones, which are obtained in theabsence of hydrogen in the chamber.

[0082] 2d. Increasing the Needle-like Tungsten Oxide Particle Length viaHigh Temperature Reaction (Stage II)

[0083] Since hydrogen was found to be indispensable for the growth ofthe needles (see 2c above), an alternative procedure for promoting theirgrowth under more controllable conditions was pursued. The basic ideawas to promote the uniaxial growth of the short tungsten suboxideneedles obtained in stage I under very low hydrogen gas concentration.For that purpose, the needles were placed in a reactor operating ataround 840° C. in a flow of (H₂/N₂) gas mixture where the concentrationof hydrogen was progressively increased to 1% (stage II). It wasbelieved that the separation between the two reactions, i.e. formationof the needle-like germs in the first place and their subsequent growth,would afford a better control of the process, enabling more uniformwhiskers to be derived.

[0084] Experiments were performed with the three types of particlessynthesized in Example 2b above in stage I by evaporation at: P_(H2O)=5,12 and 20 Torr. The results are summarized in Table 2a. The first pointto emphasize is that the two sets of short needle-like particles(evaporated at P_(H2O)=12 and 20 Torr) in contact with the gas mixture(1% H₂/99% N₂) are transformed into long tungsten oxide nanowhiskers(several microns in length) at 840° C. This is quite a large elongationconsidering the fact that the starting material consists of oxideneedles that are usually no longer than 50 nm. Moreover, the amorphousoxide nanoparticles, which are shapeless (P_(H2O)=5 Torr), are alsoconverted into very long whiskers. Consequently, the startingneedle-like morphology is apparently not relevant for inducing thegrowth of very long oxide whiskers during the annealing (stage II).Indeed, it can be summarized that the lesser the crystallinity of theprecursor (tungsten suboxide) particles, the thinner and longer are themicrolength oxide nanowhiskers obtained after stage II annealing. Themost likely explanation for this observation would be that a sublimationof a part of the tungsten suboxide particles is followed by a transportof the clusters in the gas and their condensation on some other tungstensuboxide particles, which did not sublime. For example, amorphousnanoparticles smaller than say 5 nm are likely to be more volatile thanthe larger nanoparticles (Ostwald ripening), a point discussed ingreater detail hereinbelow.

[0085] The second point to underline is the influence of the gasflow-rate (F) on the morphology of the particles, which is equivalent toa change in the pressure, (especially the partial pressure of hydrogen).This is particularly well illustrated in the series of measurements donewith the particles synthesized at P_(H2O)=12 Torr. In that case, a lowflow-rate (55 cm³ min⁻¹) generates spherical particles while a higherone (≧110 cm³ min⁻¹) brings about the growth of long nanowhiskers. Thetrend is the same whatever the starting tungsten suboxide precursor. Theflow-rate also influences the thickness of the particles, as shown bythe experiment performed with the precursor synthesized at P_(H2O)=5Torr. Indeed, in the particular case where the limit of the flow-rateallowed by the equipment was reached (300 cm³ min⁻¹), a majority of thinoxide nanowhiskers (10-20 nm in diameter) were observed instead of theusual mixture of thin and thick nanowhiskers (diameters up to 100 nm).

[0086] Since the amount of the starting oxide whiskers used for eachexperiment was quite similar from one batch to another (≅5 mg), thedifferences observed by changing the flow-rates could be attributed toeither of two parameters: the partial flow-rate of hydrogen in thereactor (partial pressure of hydrogen) or the total gas flow-rate (totalpressure). This point is particularly well expressed by the experimentperformed with the particles synthesized at P_(H2O)=12 Torr (stage I)and fired under a gas flow-rate of 55 cm³ min⁻¹ (stage II). In thatcase, whatever the fomation mechanism, the flow is so slow thatspherical or faceted nanoparticles are formed. Even the originalneedle-like morphology is not preserved in such circumstances. Note alsothat the gas flow-rate may influence the apparent temperature of the gasmixture.

[0087] It can thus be concluded that the higher the flow-rate, thehigher is the driving force to generate long and thin oxidenanowhiskers.

[0088] The morphology of the oxide whiskers obtained after annealing theparticles evaporated at P_(H2O)=20 Torr are pretty different from theprevious results, since the length and the thickness appear to besystematically limited to approximately one micron and 50-100 nm,respectively. These results indicate that, in such a case, the initialthickness and perhaps the degree of cristallinity of the needle-likenanoparticles dictates the final thickness of the elongated nanowhiskerafter annealing. Particular regard should be paid to the apex of thesewhiskers as they routinely formed perfect ninety-degree heads followingannealing. This head morphology excludes a vapor-liquid-solid (VLS)growth mode as a plausible growth mechanism.

[0089] 2e. Influence of the Hydrogen Concentration on the ElongationProcess of the Tungsten Oxide Whiskers

[0090] This point was tested by varying the hydrogen concentration inthe gas mixture. Indeed, by adding extra N₂ gas, the hydrogenconcentration was diluted from 1% to approximately 0.2%, keeping thetotal flow-rate constant. The annealing experiments (stage II) wereperformed with the precursor synthesized at P_(H2O)=12 Torr (stage I).These results are shown in Table 2b.

[0091] The first noticeable observation is that the morphology of theresultant particles of two different batches annealed at the same totalflow-rate (F_(Tot)) but at a different partial flow-rate of H₂ (F_(H2)),is different. In parallel, for two experiments, in which annealing wasdone with the same value of F_(H2) but with two different values of thetotal flow-rate (i.e. by varying the nitrogen gas flow-rate), a slightmorphological difference was observed. It is evident that bothparameters (F_(H2) and F_(Tot)) influence the morphology of theparticles (stage II), as it was previously found for the case of thetungsten filament evaporated in contact with water vapor in the chamber(stage I).

[0092] Besides this consideration, it is important to note that this setof experiments was also a useful means of determining the minimumconcentration of hydrogen required for providing the elongation of thewhiskers. Globally, it appears that decreasing the concentration ofhydrogen to 0.2% did not change drastically the morphology of theparticles, which consists of long oxide whiskers >1 μm (Table 2b). It isnoteworthy to underline the fact that the hydrogen concentration shouldbe adjusted for the given amount of WO_(3-x) particles. Indeed, theratio between the quantities of hydrogen and the starting WO_(3-x)powder must be kept constant in order to get the same kind of morphologyduring the annealing (stage II).

[0093] From this last experiment it emerges that a low concentration ofhydrogen (0.2%) is sufficient for inducing the elongation process of theoxide whiskers. Furthermore, it suggests that the sublimed phaseinvolved in the process has a stoichiometry very close to the one of thestarting precursor (WO_(2.9)).

[0094] As a conclusion of these experiments, two key parameters forinducing the oxide whisker's growth can therefore be discerned duringstage II annealing: the total gas flow (P_(Tot)) and the partial flow ofhydrogen (P_(H2)). These two factors are probably involved in thesynthesis of the WS₂ nanotubes as well, starting from the short WO_(3-x)whiskers.

[0095] 2f. Synthesis of WS₂ Nanotubes Starting From the Short OxideWhisker Precursor (Stages I+III)

[0096] The main process of the WS₂ nanotubes synthesis consists ofsulfidizing the tungsten suboxide powder in a gas mixture which iscomposed of H₂/N₂ and H₂S, where H₂ plays the role of the reducer andH₂S is the sulfidizing agent according to equation 5 (stage III):

WO_(3-x)+(1−x)H₂+2H₂S→WS₂+(3-x)H₂O  [5]

[0097] Since the growth process of the sulfide proceeds from outside in,sulfur atoms have to cross the already existing compact layers ofsulfide and therefore the oxide to sulfide conversion is diffusioncontrolled. In this way, after a few hours of reaction, all the W—Obonds of the starting material are converted into W—S bonds, leading tohollow structures without a substantial morphological change.Furthermore, since the density of WO₃ (ρ=7.16 g. cm⁻³) and WS₂ (ρ=7.5 g.cm⁻³) are quite similar, the original structure of WO₃ (and thereforeWO_(3-x)) is preserved throughout the reaction as was the case for theIF nanoparticles starting with quasi-spherical particles of WO₃.

[0098] Short WO_(3-x) needle-like particles, produced by evaporation atP_(H2O)=12 Torr in stage I, were placed in a reducing/sulfidizingatmosphere as described previously. In order to understand which factorsare responsible for the morphology of the converted sulfidized samples,only one parameter amongst three was changed at a time: the flow-ratesof H₂/N₂; H₂S and the hydrogen concentration in the gas mixture. In allthese experiments the temperature was maintained at 840° C. The datafrom these experiments are summarized in Table 3. Each Table (3a, 3b and3c) contains experiments in which one parameter was changed at a time.Comparisons could therefore be done inside each set of experiments andbetween them.

[0099] When a gas mixture with 5% hydrogen was used (Table 3a), most ofthe needle batches had similar morphologies irrespective of theflow-rate ratio (F_(H2/N2)/F_(H2S)). A TEM picture of a typical bundleof short nanotubes stemming from those batches is presented in FIG. 1band FIG. 5. The particles are hollow (FIG. 1b) or with some remainingoxide (FIG. 5) and the WS₂ layers contain very few defects. The apexesof the tubes are quite perfectly closed. The elongation of the WO_(3-x)precursors (50 nm in length and 15 nm in diameter) is not verypronounced in this case. In contrast, when the ratio F_(H2)/F_(H2S) wasvery high, long nanotubes of several microns in length could bediscerned in the samples amongst bundles of short nanotubes. However,the formation of long nanotubes was always accompanied by the presenceof metallic tungsten in their cores and, in several cases, sphericalnanoparticles of tungsten were found. Also, the number of WS₂ layers wasrather small in this case. This is attributed to fast reduction of thetungsten oxide core to the pure metal and subsequently to the slowdiffusion of sulfur through the compact metallic core (Margulis et al.,1993). Furthermore, a wide size-distribution amongst the long nanotubeswas found. Indeed, in such conditions, two types of nanotubes werepresent: “thin nanotubes”, with a typical diameter of about 20 nm andthe “thick” ones, which could reach a diameter up to 100 nm.

[0100] The variety of morphologies which appear by varying the flow-rateof forming gas (H₂/N₂) and H₂S shows that the ratio between the twogases is essential for determining the final shape of the sulfidizednanotubes.

[0101] More precisely, when the ratio F_(H2)/F_(H2S) exceeds the valueof ca. 10, either tungsten particles or nanotubes containing a tungstencore, start to appear. In that case, the hydrogen concentration in thereactor is so high compared to that of sulfur (for the amount ofprecursor taken), that the tungsten suboxide particles (WO_(3-x)) arereduced almost instantaneously into tungsten. This is anothermanifestation of the competition which occurs between the reduction andthe sulfidization processes. To avoid such an unwieldy situation, onehas to operate in a specific ratio with F_(H2)/F_(H2S)≦10. When theconcentration of H₂ in the forming gas was about 5%, short nanotubeswere produced in the range 1.4≦F_(H2)/F_(H2S)≦11 (FIG. 5) and long oneswere observed for a ratio F_(H2)/F_(H2S) above 11 (not shown).

[0102] Another aspect for the influence of the flow-rate ratioF_(H2)/F_(H2S) on the nanotubes morphology is illustrated in theexperiments where no H₂S was added to the system at all. In this case,the reactor was not flushed with N₂ prior to the experiment. Here tracesof sulfur, which remained on the reactor walls from the previousexperiment, led to the formation of long nanotubes (Table 3a). Thispoint emphasizes the fact that the ratio F_(H2)/F_(H2S) is essential forthe final morphology of the sulfidized particles.

[0103] When the hydrogen concentration in the forming gas was lowered to1% instead of 5%, the factor F_(H2)/F_(H2S) appeared not to be the onlyone responsible for the morphological changes. Indeed, two differentbatches, for which the ratio F_(H2)/F_(H2S) was kept constant (Table 3b:batches 3 and 6), gave two discernable morphologies. Besides that,careful examination of the data revealed that, decreasing the hydrogenconcentration in the gas mixture leads frequently to the formation oflong nanotubes instead of the usual short ones. This trend was even morepronounced in experiments performed with an extremely low hydrogenconcentration (less than 1%—see Table 3c). As a matter of fact, all thebatches performed with hydrogen concentration below 1% led to the growthof either a mixture of short and long nanotubes (not shown) or to almostpure phases of long nanotubes (see FIGS. 6a, 6 b). The results of Table3 lead to the conclusion that the appearance of long nanotubes dependson the ratio between the flow-rate of hydrogen and the total flow-rateof gases (F_(H2)/F_(Tot)).

[0104] It emerges therefore, that in order to achieve the formation oflong nanotubes, two flow-rate ratios have to be carefully controlled:the ratio F_(H2)/F_(H2S) and F_(H2)/F_(Tot). The conditions required forproviding long nanotubes as a majority phase in a reproducible mannerare consequently the following:

0.5≦F _(H2) /F _(H2S)≦4.5 and 0.002≦F _(H2) /F _(Tot)≦0.007

[0105] To obtain homogeneous phases consisting of purely long nanotubeswithout tungsten in their core, the conditions are even morerestrictive:

1≦F _(H2) /F _(H2S)≦2.2 and 0.005≦F _(H2) /F _(Tot)≦0.007

[0106] It emerges from all these results that a careful control of thesynthesis parameters leads to a specific and desireable morphology ofthe nanotubes.

[0107] 2g. Synthesis of WS₂ and WSe₂ Nanotubes Starting With theElongated Oxide Whiskers (Stages I+II+III)

[0108] The purpose of this last study was to explore the possibility tosynthesize long WS₂ nanotubes from the already existing long oxidenanowhiskers obtained in stage II. The long oxide nanowhiskerssynthesized from the short whiskers (see Example 2b above) were placedin a reducing and sulfidizing atmosphere without taking specificattention to the ratios F_(H2)/F_(H2S) and F_(H2)/F_(Tot). All theattempts led to the formation of long nanotubes, although the degree ofcrystallinity of the nanotubes was not perfect. The WS₂ layers containedplenty of defects (FIGS. 7a, 7 b) and rather quite large proportion ofthe nanotubes was not totally closed at their apex (not shown).

[0109] The two-step method of the invention may be more difficult tocontrol, but it gives very satisfactory results. The three-step methodof the invention lends itself to the synthesis of nanotubes from relatedcompounds, such as WSe₂ or mixed WS₂/WSe₂ using preprepared long oxidenanowhiskers as a precursor.

[0110] Indeed, WSe₂ nanotubes were prepared by heating selenium ingot at350° C. downstream of the main reactor, which was heated to 760° C.Forming gas (1% H₂/99% N₂−110 cm³ min⁻¹) was provided in this case. Theresulting WSe₂ nanotubes were quite perfect in shape.

Example 3 Growth of the WS₂ Nanotubes in the Two-step Process

[0111] From the present measurements, one can visualize the growthprocess of the encapsulated nanowhisker as depicted in FIG. 8. At thefirst instant of the reaction (FIG. 8A), the asymmetric tungsten oxidenanoparticle reacts with H₂S and forms a protective tungsten disulfidemonomolecular layer, prohibiting coalescence of this nanoparticle withneighboring oxide nanoparticles. Simultaneous condensation of (WO₃)_(n)or (WO_(3-x).H₂O)_(n) clusters on the nanowhisker tip and reduction byhydrogen gas leads to growth of the sulfide-coated oxide nanowhisker.This process is schematically illustrated in FIG. 8B. Note that duringthe gradual reduction of the oxide core, the CS planes in the oxidephase rearrange and approach each other until a stable oxide phase W₃O₈is reached (Iguchi, 1978). In FIG. 8C, {010} is the growth axis of theW₃O₈ whiskers. This phase provides a sufficiently open structure for thesulfidization to proceed until the entire oxide core is consumed.Further reduction of the oxide core would bring the sulfidizationreaction into a halt (Margulis et al., 1993). Therefore, theencapsulation of the oxide nanowhisker, which tames the reduction of thecore, allows for the gradual conversion of this nanoparticle into ahollow WS₂ nanotube.

[0112] Naturally, the elongation of the oxide nanowhiskers requires areservoir of (WO₃)_(n) clusters in the vapor phase. Conceivably, thevaporized oxide clusters react only very slowly with the H₂ and H₂Sgases, which would otherwise hamper the rapid growth of the oxidenanowhisker. The termination of the nanowhisker growth occurs when thesource of tungsten oxide is depleted and the vapor pressure of the oxideclusters diminishes below a critical value. In this case thesimultaneous reaction of tip growth/reduction and sulfidization cannotbe maintained and the outer sulfide layer of the encapsulate completelyenfolds the oxide tip. In fact, this is the reason that the WS₂nanotubes are almost the sole phase comprising the mat of FIGS. 2 and 6.This mechanism entails that part of the oxide nanoparticlespredominantly elongate through tip growth, while the rest of the oxidenanoparticles furnish the required vapor pressure for the tip growth ofthe former population, and they slowly diminish in size (Ostwaldripening). The oxide vapor can not condense and stick onto the sulfidewall of the encapsulated nanowhiskers and therefore no thickening of thenanotubes or their bifurcation, is observed.

Example 4 WS₂ Nanotubes as Tips for Scanning Probe Microscopy

[0113] WS₂ nanotubes were attached to microfabricated tips of an atomicforce microscope (AFM) by transferring adhesive from carbon tape to a Sitip, then pulling off nanotube bundles with this tip from a mat ofnanotubes prepared on a different area of the tape (Dai et al., 1996). Aportion of this mat was glued to the Si tip, of which the longestnanotube served now as the new tip. Scans on a Ti tip calibrator (Westraet al., 1995) and subsequent blind reconstruction (using algorithmdeveloped by A. Efimov, obtainable at http://www.siliconmdt.com) of tipshape gave tip width of 16 nm for the last 100 nm of the tip length. Inorder to demonstrate the capabilities of these tips for investigatingdeep and narrow structures, they were used to image a line structure ofdepth 670 nm and varying linewidth. As seen in FIG. 9, the nanotube tipsperform significantly better than microfabricated sharp Si tips. Whilethe WS₂ nanotube tip follows the contour of even the finest replica andreaches its bottom, the commercial Si tip is unable to delineate thereplica correctly due to its slanted edge. Also, the Si tip is unable tofollow the sample contour very smoothly (see for example FIG. 9C),because the surface of this tip is not passivated and therefore stronginteraction with the substrate at close proximity is unavoidable.Thinner or single walled nanotubes may not be useful for suchapplications because of their small spring constant toward bending.Also, due to its sandwich S—W—S structure, the WS₂ nanotubes areprobably stiffer than their carbon analogs. In contrast to carbonnanotubes, the present nanotubes can be easily sensitized by visible andinfra-red light and therefore show promise as a selective probe fornanophotolithography.

REFERENCES

[0114] 1. Ajayan, P. M.; Stephan, O.; Redlich, P h.; Colliex, C. Nature1995, 375, 564.

[0115] 2. Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.;Cohen, M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966.

[0116] 3. H. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert, R. E.Smalley, Nature 1996, 384, 147. See also, J. H. Hafner, “Nanotube tipsfor SFM”, http://cnst.rice.edu/mount/html.

[0117] 4. Feldman, Y.; Wasserman, E.; Srolovitz, D.; Tenne, R. Science1995, 267, 222.

[0118] 5. Feldman, Y.; Frey, G. L.; Homyonfer, M.; Lyakhovitskaya, V.;Margulis, L. Cohen, H.; Hodes, G.; Hutchison, J. L. and Tenne, R. J. Am.Chem. Soc. 1996, 118, 5362.

[0119] 6. Y. Feldman, V. Lyakhovitskaya, R. Tenne, J. Am. Chem. Soc.1998, 120, 4176.

[0120] 7. G. L. Frey, R. Tenne, M. J. Matthews, M. S. Dresselhaus, G.Dresselhaus, J. Mater. Res. 1998, 13, 2412.

[0121] 8. Glemser, O.; Weidelt, J.; Freund, F. Z. Anorg. Allg. Chem.1964, 332, 299.

[0122] 9. F. D. Hardcastle, I. E. Wachs, J. Raman Spectrosc. 1995, 26,397.

[0123] 10. Hershfinkel, M.; Gheber, L. A.; Volterra, V.; Hutchison, J.L.; Margulis, L.; and Tenne, R. J. Am. Chem. Soc. 1994, 116, 1914.

[0124] 11. J. A. Horsley, I. E. Wachs, J. M. Brown, G. H. Via, F. D.Hardcastle, J. Phys. Chem. 1987, 91, 4014.

[0125] 12. E. Iguchi, J. Solid State Chem., 1978, 23, 231.

[0126] 13. Iijima, S. Nature 1991, 56, 354.

[0127] 14. Margulis, L.; Salitra, G.; Talianker, M.; Tenne, R. Nature1993, 365, 113.

[0128] 15. T. Miyano, M. Iwanishi, C. Kaito, M. Shiojiri, Jap. J. Appl.Phys. 1983, 22, 863.

[0129] 16. Remskar, M.; Skraba, Z.; Cleton, F.; Sanjines, R.; Levy, F.Appl. Phys. Lett. 1996, 69, 351.

[0130] 17. Remskar, M.; Skraba, Z.; Regula, M.; Ballif, C.; Sanjines, R.and Levy, F. Adv. Mater. 1998, 10, 246.

[0131] 18. Remskar, M.; Skraba, Z.; Ballif, C.; Sanjines, R.; Levy, F.Surf. Sci. 1999a, 435, 637.

[0132] 19. Remskar, M.; Skraba, Z. Appl. Phys. Lett. 1999b, 74, 3633.

[0133] 20. Sloan, J.; Hutchison, J. L.; Tenne, R.; Feldman, Y.;Tsirlina, T.; Homyonfer, M. J. Solid State Chem. 1999, 144, 100.

[0134] 21. Tenne, R.; Margulis, L.; Genut, M. and Hodes, G. Nature 1992,360, 444.

[0135] 22. K. L. Westra, D. J. Thomson, J. Vac. Sci. Technol. 1995, B13, 344. Obtainable from General Microdevices, Edomonton, Alberta,Canada.

[0136] 23. Zelenski, C. M.; Dorhout, P. K. J. Am. Chem. Soc. 1998, 120,734. TABLE 1 Comparative d-spacing data between the needle-likeprecursors and the tetragonal WO_(2.9) reported by Glemser et al., 1964.The d_(hkl)-spacings were obtained from the ED ring pattern of the oxideparticles. A TiCl pattern was used as a standard reference. Oxideprecursors Tetragonal WO_(2.9) Irel d_(hkl) (Å) Irel d_(hkl) (Å) hkl100  3.752 100 3.74 110 20 3.206 20 3.10 101 80 2.640 80 2.65 200 302.184 30 2.20 201 — 10 2.02 211 30 1.878 30 1.88 220 10 1.703 10 1.78300 60 1.558 60 1.67 310 50 1.153 50 1.53 311 — 10 1.33 222 — 10 1.25330 — 10 1.17 322

[0137] TABLE 2a P_(H2O) Flow 1% H₂/99% N₂ Morphology of the (Torr) (cm³min⁻¹) particles 5 110 L_(ox−t) & L_(ox−T) (>1 μm) 5 200 L_(ox−t) &L_(ox−T ·) (>>1 μm) 5 300 L_(ox−t) >> L_(ox−T) (>>1 μm) 12 55 S + F 12110 L_(ox−t) & L_(ox−T) (>>1 μm) 12 200 L_(ox−t) & L_(ox−T) (>>1 μm) 20110 L_(ox−T) >> L_(ox−t) (≅1 μm) 20 200 L_(ox−T) >> L_(ox−t) (≅1 μm)

[0138] TABLE 2b Flow Flow P_(H2O) 1% H₂ N₂ F_(tot) F_(H2) % H₂ =Morphology of (Torr) (cm³ min⁻¹) (cm³ min⁻¹) (cm³ min⁻¹) (cm³ min⁻¹)F_(H2)/F_(tot) the particles 12 110 100 210   1.1 0.52 L_(ox−t) &L_(ox−T) (>>1 μm) 12 200 100 300 2 0.73 L_(ox−T) >> L_(ox−t) (≅1 μm) 12200  50 250 2 0.88 L_(ox−t) & L_(ox−T) (>>1 μm) 12 200  20 220 2 1  L_(ox−t) & L_(ox−T) (>>1 μm) 12 100 200 300 1 0.33 L_(ox−t) & L_(ox−T)(>>1 μm) 12  50 200 250   0.5 0.2  L_(ox−T) >> L_(ox−t) (≅1 μm)

[0139] TABLE 3a Flow of 5% H₂/95% Flow of N₂ H₂S F_(H2)/ F_(H2/N2)/Morphology of the (cm³ min⁻¹) (cm³ min⁻¹) F_(H2S) F_(H2S) samples 110 41.375 27.5 Sh 110 2 2.75 55 Sh 110 1 5.5 110 Sh 110 0.5 11 220 Sh with Winside 110 0* L_(T) with W inside + W 200 2 5.5 110 Sh 200 1 10 200 Shwith W inside 200 0.5 20 400 (L_(t) & L_(T) + Sh) with W inside 55 21.375 27.5 Sh

[0140] TABLE 3b Flow of 1% H₂/99% Flow of N₂ H₂S F_(H2)/ F_(H2/N2)/Morphology of the (cm³ min⁻¹) (cm³ min⁻¹) F_(H2S) F_(H2S) samples 110 20.55 55 Sh + 2H − WS₂ 110 1 1.1 110 Sh + L_(t) & L_(T) + IF + 2H − WS₂110 0.5 2.2 220 Sh 110 0.3 3.7 314 Non defined shape + IF + Sh 200 2 1100 Bad encapsulation 200 1 2 200 Sh + very few L + IF + 2H − WS2 2000.5 4 400 (Sh + L_(t) & L_(T) + IF) with W inside 55 2 0.275 27.5 2H −WS₂

[0141] TABLE 3c Flow of Flow of Flow of 1% H₂/99% N₂ N₂ H₂S % H₂ =Morphology of (cm³ min⁻¹) (cm³ min⁻¹) (cm³ min⁻¹) F_(H2)/F_(H2S)F_(H2)/F_(tot) the samples 200 100 1 2 0.66 Sh + L_(t) 110 100 1 1.10.52 Sh + L_(t) 100 200 1 1 0.33 Sh with W inside + L_(t) & L_(T)  50200 1 0.5 0.2  Sh + L_(t) 100 200   0.5 2 0.33 (Sh + L_(t) & L_(T)) withW inside 110 100   0.5 2.2 0.52 L_(T) & L_(t) + Sh 200 100   0.5 4 0.66L_(T) & L_(t) >> Sh with W inside 200 100 2 1 0.66 L_(t) & L_(T) >> Sh110 100 2 0.55 0.52 Sh >> 2H—WS₂

What is claimed is:
 1. A method for preparation of tips for scanningprobe microscopy which comprises: a) transferring adhesive from carbontape to a microfabricated Si tip; and b) pulling off bundles of atransition metal chalcogenide nanotubes with said tip from a mat of longnanotubes prepared on a different area of the tape, wherein said longnanotubes have a size of 0.2-20 μm or greater, and are obtained by bulksynthesis of long nanotubes of transition metal chalcogenides from atransition metal material, water vapor and H₂X gas or H₂ gas and Xvapor, wherein X is S, Se or Te and synthesis, comprising: a) eitherheating a transition metal material in the presence of water vapor in avacuum apparatus or electron beam evaporating a transition metalmaterial in the presence of water vapor, at a preselected pressure, toobtain nanoparticles of the transition metal oxide as long as 0.3microns; and b) annealing the transition metal oxide nanoparticlesobtained in step (a) in a mild reducing atmosphere with a H₂X gas or H₂gas and X vapor, wherein X is S, Se or Te, at a suitable temperature,thus obtaining said long nanotubes of the transition metal chalcogenide,said nanotubes.
 2. A method according to claim 1, wherein saidtransition metal chalcogenide is WS₂ and/or WSe₂.